This is a lie. It's simply not true. Grab any retail box of Snow Leopard, and you'll see it doesn't say ANYWHERE that it's an upgrade. Apple fanatics might think it is, but wishful thinking does not make it so.

I've bought more Mac OS X boxes than you can count, and not one of them has the word "upgrade" on it, and ALL of them install just fine on their own. Nor has any shop assistant ever informed me that it is, in fact, an upgrade. As such, I'm buying a normal piece of software, and I can do whatever the hell I want with it.